Our focus within this project is to integrate superconducting and acoustic quantum circuits; thereby bringing together the quantum non-linearity found in Josephson-junction based microwave electrical qubits and the ultra-long coherence time of microwave acoustic resonators recently demonstrated in the Painter group. Our proposed approach integrates planar superconducting quantum circuits (SQCs) with planar thin-film phononic and optomechanical crystal (OMC) circuits, enabling a new class of quantum transducers and memory elements key to realizing a network of superconducting quantum nodes. In addition, the connectivity of SQCs to microwave-frequency acoustic phonons provides a natural interface to both optical photons for long-range networking via optical fiber networks and color-center defects for atomic-scale networking of electron and nuclear spins.

Schematic: Microwave-to-optical conversion via mechanical intermediary.

Unlike prior work integrating lossy microwave-frequency acoustics with superconducting devices, our proposed hardware platform provides a dramatic reduction (X100) in the physical size of circuit elements, decreased cross-talk between elements, and the potential for longer (X10,000) coherent storage time of quantum information. Based upon phononic crystals, our approach uses the geometric patterning of a released thin-film surface layer, such as the silicon device layer of silicon-on-insulator (SOI), to guide, trap, and diffract microwave acoustic waves with exquisite control, negligible loss, and small chip real estate. Coupling between acoustic waves and electrical elements in the superconducting microwave circuits is realized using an additional thin layer of piezoelectric material. By selective patterning and removal of the ultra-thin layer of piezoelectric material, acoustic and electrical losses can be minimized while allowing for local coupling between acoustic and electrical fields. Upconversion of acoustic waves into optical photons will be provided by radiation pressure (an elasto-optic nonlinear effect), which can reach extreme levels within nano-scale silicon optomechanical cavities. The main activities of our research can be broken down into the following main categories described below: (i) physics and engineering of ultra-coherent nano-acoustics at microwave frequencies, (ii) development of phononic quantum memory and quantum transducer elements for integration with SQCs, and (iii) development of OMC optomechanical transducers for coupling to SQCs and integration into a quantum optical network.